I mean... The difference between history and the future is you can't change history. There is no other way it happened; "could have happened" is the realm of speculative fiction.

If we want to turn the stories of the past into questions about what we could do differently right now, that's an interesting conversation to me. "But what if the South had just decided not to get into a shooting war with the North?" is fodder for a stack of books on the "New Fiction" table at Barnes and Noble but not much more.

Turning the lens to the present: I think it is worth noting that decades of negotiation, political horse-trading, and compromises had been attempted prior to the breakout of the War. It isn't that talking wasn't tried, it's that one side got tired of having the conversation every single generation (and were perceiving that the zeitgeist were turning against their position). So one useful question is "What are the divisions in this era that mirror the kind of irreconcilable difference that was 'a nation half-slave and half-free?'" One candidate I could suggest is the question of gun control; I suspect it is not, as practiced in the US, a topic where people can agree to disagree, the Constitutional protection (and judicial interpretation of it) distorts the entire conversation, and I think there's real nonzero risk of one side responding to a sea-change in the zeitgeist conversation with violence.

Which side, I do not yet predict. A major ingredient in the slavery debate was existential fear (the belief in the South that a freed black population would form either a power bloc that would destroy its former masters politically or vigilante posses that would do violence to their former masters). It's one of the reasons John Brown's raid was so terrifying to the Southerners because Brown was a white man who committed political violence in the name of ending slavery; they perceived him as a signal that the North was done talking (even though he was not acting as an agent of the government). In modern America? A lot of Americans are terrified of random gun violence. That kind of terror lowers the bar on willingness to commit violence, because the survival drive runs hot. And, similarly, gun owners are terrified that the government could strip their capacity for self-defense from them and they'd then be vulnerable to violence they could not defend themselves from.

If you're looking for a lesson from the past on how to diffuse such a volatile situation... Unfortunately, I don't think the story of the Civil War will give it to you. That's a story of failing to diffuse it.